Differential spring balance tab



Dec. 4, 1951 H. o. WENDT 2,577,439

DIFFERENTIAL SPRING BALANCE TAB Filed oct. 18, 194'.7 2 SHEETS-SHEET 1 @mmf/Em A TTORNEY Dec. 4, 1951 Filed Oct. 18, 19 4'? TAB DEFLECTION IN DEGREES H. O. WENDT DIFFERENTIAL SPRING BALANCE TAB @50 FIG? n: MAXIMUM ALLowAeLE m40- D.

53o- &FAB L 2O @WL- 5 MINIMUM ALLowABLE E' PLAIN GE AR E D 1A z o O O o l l Ioo 20o 30o 40o INDIcATED AIRSPEED -M.I=.H.

|20- FIG. 6 g g Ioo- ,b o l? L Z eol O 6o- E MAXIMUM ALLowABLE n be l 4o- D: O 2o- O CONTROL SURFACE DEFLECTION 30 FIG. 5

Q. oV0 2: Q, 1 9%' |O 4 y OV f Q` Les O/ 3/ INVENTOR. O l5 LBS BY HAROLD O. WENDT l O +IO +2'0 +30 CONTROL SURFACE DEFLECTION ATTQRNEY Patented Dec. 4, 1951 'DIFFERENTIAL .SPRINGBALANCE TAB- Harold Y0. .Wendt, Columbus, Ohio, Vzisnsignor to Curtiss-.Wright Corporation, a corporation of Delaware n .Application-0ct0ber18, 1947,.:-Sera1-Nont80301 9 claims. (clam-sz) This-invention :relates ,to airplane .control fl'nens of the type:Winer-eineJ I,tab ishingd to v.the @railing-ledge of .acontrolairfoil forproviding afi-n edynamic balance.

risually tabs vor ,this .kind are geared to kthe .airplane structure in ,-such vmanner that deflection of theftab rrelati-vefto the control arfoilis approximately proportional totheioontrol airioils tdeflection. n ,some instances ,means have vbeen@provided whereby thisjproportion, :or balance :,ratio, :may be adjusted to n.dierent Yvalues by :the Apilot vso `.that Lmore or Aless aerodynamic Arelancemay ite-provided. As applied to large v,airplanes :the geared :tab -arrangements of this general fkiiid, Yifdesignedor adjusted; so -thattthe .pilot g'ontrol 4forces do not `become excessive at l:ver/y high `angles-of.deflectionlofthe control air- ;oil, vhave the disadvantage rthat rythe rvcontrol force-required to ,produce a lgvenangular -acceleration of the ,-airplane, for g loadis.so low i :that 'the .pilot :may not obtain the minimum -defgree vof -control -feel that is `regarded as necessary for lsafe airplaneoperation. `AConverselyif with the Vgeared balance Vtab there is suiiicient oontrolforce :per g then the control force will bef-excessive at `high angles of ,control .airfoil Ydeflection whichare often.necessary,..at relatively low alrspeeds.

The vpresent .invention provides a differential spring 4balance ,arrangement whereby the tab ,deeetion .isproportionate both to controlairfoil r-deection ,and to control force. As with ,con- .rentional 4geared balance tabs the .tab effect .is small at Small vartigli-Ts of y controlairfoil deection,=but by the response of thefpresentarrange- .ment .toecontrol forces the `eiect .of .the talov at high angles vof i. control air-foil deflection .may ,be .either .large or small depending upon ,Whether :thecontrol force is vhiglfror low. Accordingly with this arrangement the control .forceseat high angles of deflection of .the .control surface may he kept `within .the .desired limits .while .at .the same time 7the -control Iforce'per .g may ybe Vsuiicient to 4aiord :the ,pilot the A11ecessary--de greeofcontrol feel.

The present invention di'ers .substantially .from the `conventional spring tab varrangement dn ,which the tab has an`unyielding vconnection with .the pilot `operatedcontrol systerrrand the .Qontllol .-airfoil .is connected theretoby a .spring unit. With rsuch anfarrangement the tab deection varies with pilot control force and -has no direct relation to .the degree .of kdeflection fof the feontrol'airfoil. Such -an arrangement :is effective from thestandponl? 9i reducing pilot ,control 2 ronces., but it. possesses. the undesirable characteristic Vthat-at higher airspeedS` the control iorcepr ,g decreases tosuch an extent .that the vpilot ,may no longer obtain the desired degreeoicontrolfeel o Objects and advantages of the lpresent invention additional to.. those referred to ,hereinbefore Will appear from ,the following descriptionymade .with lreerence .to the accompanying drawings, wherein:

Figure lis a, .pers pective view of the dili'erential balance tab'linkage as ,applied toa .tab associated'with'an airplane stabilizer .and elevator, vthe tab and the stabilizer-elevator assemblyheing showninphantom;

Figure 12,;is a side elevation ofthe elements shown in'Figure 1, 'thejull line .position representing the condition of the parts with the elevator .in neutral position and with no control .force applied, andthe dotted line position repre:- senting,l an up-,elevatorv no #control-force condi.- Ation;

' Figure .'3 is a viewsmilar to Eigurejzbutillustrating in full lines ja down-elevatorl highcontrol-force .condition andjn dotted lines an bip-elevator high-,control-foroe condition ;A

. Figure 4 ,is `a longitudinal sectional view through `the spring unit shown Vin .the preceding views;

Ehgure ..5 .is a graph showing `the ,tab deflection .versus .controlsurface.,deliection Afor ,Various control 'forces .with .the 'tab arrangement Qf.. t he present .invention applied to an. .airplane .chosen as .anexample Figure 6 .is agraphrshowing .thecontrol force versus control surfacerelationship-Wththe present .tab-.arrangement for .the .example airplane .in `comparison with Vthe corresponding .relationships for the sameairplane .without.. any tahand .also with. a conventional geared tab; and

Figure 7 .-.is .a graphlshowing a similar com,- Tparison for :the icontrol iorce f.per em-versus airispeedrrelationship.

=As wshown in ,Eigures l vand 2, -lthe control .airfoil orfelevatorzi is hinged upon fang-axis desiggnated flfl .toa part'ofrthe .airplane.structure,fin Vthis case to ythe trailing edge -of a 4horizontal ystabilizer fl-2, Yand 4the .aerodynamic balance .tab i3 Aispivoted-about-anfaxis 1.4 tofthe trailingwedge of the --elevaton .For-.controlling the .Velevatorffl ,suitablemirage-including .push-191111 rod ,l5 is `extended vrfrom 1the lpilot .operable control stick or control .wheelecolumn IRod =I 5 .is pivoted l at .l5 topneearmfofva rst yhellcrank..lever ,l-'l which riveted. at :1.8. toga fbragkeielaedi? @he @1- vator and whose other arm is pivoted at I9 to a connecting link 2|. to one arm of a second bellcrank 23.

The bellcrank 23 is pivoted at 24 to a bracket that is rigid with the elevator, and its other arm is nivoted at 25 to another connecting link 26. This connecting link is pivotefA at 21 to each of links 2R and 29. Link 28 extends into the stabiliver where it is pivoted at 3| to the airplane structure. and its function is to support pivot 21 for movement relative to the airplane structure I2. Link 29 is the control link for tab I3 and it is nivoted at 32 to a control horn on theI tab.

Pivoterl at 33 to the elevator is a spring cartridge unit 34 comprising a, tubular casing 35 with adiustahle screw-threaded end closure cars 38. Though the latter extends slidably a rod 31 which. is pivo'red at I6 to rod I5 and to bellcrank I1. Springs 38 within the cartridge casing act unon a crass-head 39 that is secured to the rod 31 and is siidable within the casing. It wm be seen that the springr cartridge unit serves to ela stically restrain movement of the bellcrank I 1 relative to the elevator. and together with the bellcrank serves as a connection, effective between the control means I and the elevator, that is capable of yielding under control forces.

It will be observed that, in the neutral condition of the elastic means depicted in Figures l and 2, the nivot connection 21 of links 26, 28 and 29 is substantially coincident with the pivot axis I I of the elevator. As long as no control pressure is applied. the tab I3 will remain in its neutral position with relation to the elevator, even though the latter is deflected. An example of this deilected conditionis the up-elevator position shown in dotted lines in Figure 2.

Unon the application of a control force which deflects the elastic means in either direction the bellcrank 23 will be swung clockwise (as viewed in Figures 1 to 3) relative to the elevator, displacing the pivot 21 from the elevator hinge axis II. If the elevator is in its neutral position wherein the links 28 and 29 are in nearly straight line relation as will appear from Figure 2, such displacement of pivot 21 will not appreciably move the tab I3 relative to the elevator. But with deflection of the elevator in either direction from its neutralposition, the displacement of pivot 21 from hinge axis II will cause an opposite deiiection of the tab relative to the elevator, to provide an aerodynamic vbalancing effect. It will thus be seen that the degree of tab deflection will depend upon the degree of elevator deilection and also upon the amount of control force applied through the pilot's control system.

With the tab operating linkage arrangement shown in the drawings applied to an airplane chosen as an example the degree of tab deflection per degree of control surface deflection is shown in Figure 5 for various control forces applied to the control wheel of the airplane. It will be seen from this example that the ratio of tab deflection to control airfoil deflection increases rapidly with increase of control pressure. As is shown, at a 20 control airfoil up-deflection, with a 60 pound control force, the tab down-deflection will be approximately 20 or approximately twelve times as great for a 15 pound control force with which the tab down deflection will be only about 2.

.The advantageous effects of the present tab arrangement as compared with other arrangements is shown graphically in Figures 6 and '1. The solid line curve in Figure 6 shows the relation- "ship of control force to control surface deflection The latter is pivoted at 22.

when the control surface chosen as an example is provided with the tab arrangement of the present invention and the airspeed is such that at a. maximum control surface deflection of 27 the control force is fty pounds, this control force being assumed to be the permissible maximum. The short dotted line curve of Figure 6 shows the control forces under the same conditions when the same airfoil is provided with a conventional geared tab that is geared so that the same maximum allowable control force of fty pounds is re- .quired at maximum control surface deflection.

The long dotted line curve of Figure 6 shows the relationship under the same conditions for the same airfoil without any tab. It will be noted that when no tab is employed the required control force exceeds the permissible maximum long before the maximum airfoil deflection is obtained.

In Figure '1 is shown the control forces, required to produce an acceleration force on the airplane of one gravity, with each of the three tab arrangements indicated in Figure 6. It will be seen that this control force per g is well above fifteen pounds, chosen as the permissible minimum, for the control airfoil when provided with no tab and also when provided with the tab system of the present invention. However with the conventional geared tab the control force per g is well below the required minimum.

It will be understood that the values of control forces, deflection angles, and airspeeds shown on the graphs, Figures 5 to '1, are merely examples computed to illustrate the principles involved, and that these values will differ for each different airplane design. However, the graphs illustrate that the present tab arrangement pro'- .vides a means whereby the control forces may be kept within established limits which would be impossible without any balance tab or with the conventional geared balance tab.

Another advantage, inherent in the present tab operating link arrangement, resides in its anti-flutter characteristics. As will be seen from Figure 2, the links 23 and 29 assume a nearly straight line relation to each other when the elevator is in neutral or nearly neutral position, which will almost invariably be the case at very high airspeeds. As a result the aerodynamic loads on the tab will produce only very small vertical loads on pivot 21. Furthermore in the no-control-force condition of the linkage, wherein as shown in Figure 2 the pivot points 22, -I9 and I8 are substantially in straight alignment, or dead center relationship, vertical loads on pivots 21 will not be transmitted to the control link I5. Accordingly the operation of the linkage is substantially irreversible when at and near neutral condition of the controls, and will therefore discourage flutter from developing in the system at high airspeeds.

It will be understood that while described with particular relation to an airplane elevator the inventionis equally applicable to rudders, aile'r'- ons, elevators and other kinds of control sur-"- faces. It will be understood further that the structure and arrangement of parts shown and described herein are merely by way of illustrating the inventive principles that are involved, and that these principles may be otherwise embodied without departing from the spirit of my invention or from the scopel of the appended claims.

I claim:

l. In a control system for an airplane having xed airfoil structure and a control airfoil hinged to Vthe xed structure, the combination theretab? andi a 'second'- linkh pivoted te said stru'starei` saic'f '1in-ks beingpivotedfw each other 'a d te irst connecting: link upon-a pivot'l axi'stliat'mayf be moved into substantial 'coincidence witlithe controlairfol hinge axis, iir'stA andc second belli-r cranks pivoted to'theairfoil, theris'tbelicrank having one arm thereof pivoted to' a'pusl'i-'epull control'- element' and' the other `arm-thereofpi'v otally' conne'cte'dby" aI secondi connecting link' tof one` arm ofthe" second: bell'crank," the" other'arm df th'esecnd' bellcranlf being llilte'd'l to Sillt lir'st connecting link, and elastic means' for'iyieldably' supporting the firstl bellc'rank in the neutralF pfo" sition, thereof relative `to the control? airfoil i'n which said linkl pivotaxis is substantially coincident with .thecontrol airfoil hinge axis, said second connecting link havihg'the'pivots theref irr- 'Substa'ntially' l'strai'g'll'lalgnine'i- Witll'f'the pivot of the first bellcrank wh'erri'n 'said'nutral position to'substantialiy'preventitheitransmissicn f of forcesirom the tabto the control link.v

, 2. Inj. control system fran airplane'.liafi7i-iijgfv therewith comprising:atabfhing'dito the trailing first connecting link upon a pivot axis that may be moved into substantial coincidence with the control airfoil hinge axis, iirst and sec-V ond members pivoted to the airfoil, the first member having one pivot connection to a pushpull control element and another pivot connection to a second connecting link, the second connecting link being pivoted to the second member, said rst connecting link being pivoted to the second member, whereby upon pivotal movement of the first member by said push-pull control element the link pivot axis will be moved, and elastic means for yieldably supporting the ilrst member in the neutral position thereof relative to the control airfoil in which said link pivot axis is substantially coincident with the control airfoil hinge axis.

3. In a control system for an airplane having a control airfoil hinged thereto, the combination therewith comprising a tab hinged to the trailing edge of the airfoil, a first link pivoted to the tab and a second link pivoted to said structure, said links being pivoted to each other upon a pivot axis that may be moved into substantial coincidence with the control airfoil hinge axis, a con# trol element and a differential linkage connecting the element with the airfoil and with said links, said diierential linkage upon movement of the control element in either direction eiecting movement of the airfoil or displacement of said pivot axis in one direction from coincidence with the control airfoil hinge aids or both airfoil movement and pivot axis displacement, saiddifferentlal linkage including elements that are in substantially dead center relationship when said axes are in substantial coincidence to substantially prevent the transmission of forces from the tab tothe control link, and elastic means for resisting such displacement of said link pivot axis.

4. In a control system for an airplane having a control airfoil hinged thereto, the combination therewith comprising a tab hinged to the trailing edgegfofgthe airfoil, a rst link pivoted to the tab and aisecond link pivoted to the airplan'esaid links being pivoted to each other and to a first connecting link upon a pivot axis that may be moved into substantial coincidence with the conthe tab to the-controlling.

-.V In-l a; control" system for" an" airplane laving a' control" airfoil hingedi thereto, the combination. therewith comprising atab'ihinged to` the" tif-ailing? ed'ge-ofthe'airfoil, a rstlink pivoted tfo' the tabl and afsecon'd link pivotedtotheair'plae', 'said-1 linksebeing pivotedto 'each' otherand tei 'a' conn'e'ctingV link upon 'alpivot 'axis' that-may b'l movedv` inte'substantialcoincidenc with the?` control"r airfoilhinge axis;- a control element a dl'i differential' linkage connecting the? element" with? the" airfoil'and" said links, said*- differential linke#- age'fupon movementy ofj the control eleri'ie'r'it eitherv direction: effecting nvoifementf the a" foil? ini ar` corresponding 'directioni'orl d-isplaem'erlt of said link pivot axis in one direction from coincidence with the control airfoil hinge axis or both such airfoil movement and link pivot axis displacement, and elastic means for resisting such displacement of said link pivot axis.

6. In a control system for an airplane having a control airfoil hinged thereto, the combination therewith comprising a tab hinged to the trailing edge of the airfoil, a rst link pivoted to the tab and a second link pivoted to the airplane, said links being pivoted to each other and to a rst connecting link upon a pivot axis that may be moved into substantial coincidence with the control airfoil hinge axis, a control element and a diierential linkage connecting the element with the airfoil and said links, said differential linkage upon movement of the control element eiecting movement of the airfoil or displacement of said link pivot axis from coincidence with the control airfoil hinge axis or yboth airfoil movement and link pivot axis displacement, and elastic means for resisting such displacement of said link pivot ax1s.

7. In a control system for an airplane having a control airfoil hinged thereto, the combination therewith comprising a tab hinged to the airfoil, a tab operating link having a pivot connection to a member that is mounted for movement relative to the airplane, the axis of said pivot connection in one position of said member being substantially coincident with the control airfoil hinge axis so that the tab will have no substantial movement relative to the control airfoil upon movement of the latter, the tab being moved relative to the control airfoil upon movement of the latter when said pivot connection is displaced from the axis of the control airfoil hinge axis, a control element, diirerentiai means connecting the control element with the airfoil and with said member in such manner that upon movement of the control element in either direction from a neutral position there will be movement of the airfoil in a corresponding direction ormovement of said member in only one direction from said onepcsition thereof or both of said last-mentioned movements, and means for elastically holding said member in said one position thereof.

8. yIn a-control system for-an airplaney having a control airfoil hinged thereto, the combination therewith comprising a tab hinged to the airfoil, a tab operating link having a pivot connection to a member that is mounted for movement relative to the airplane, the axis of said pivot connection in one position of said member being substantially coincident with the control airfoil hinge axis so that the tab will have no substantial movement relative to the control airfoil upon movement of the latter, the tab being moved relative to the control airfoil upon movement of the latter when said pivot connection is displaced from4 the axis of the control airfoil hinge axis, a control element and diierential means connecting the control element with the airfoil and with said member in such manner that upon movement of the control element from a neutral position there will be movement of the airfoil or movement of said member or both of said lastmentioned movements, and means for elastically holding said member in said one -position thereof. 9. In a control system for an airplane having a control airfoil hinged thereto, the combination therewith comprising a tab hinged to the airfoL' that the tab is deflected relative to the airfoil in, response to and-substantially n'proportionto the deiiection of `the airfoil relative to theairplane but in the .opposite direction, means operi.-v atively connected to said link for adjusting thev location of the joint of said link toward and away from the airfoilhinge axis to vary the ratio of tab deflection to airfoil deflection, meansfor actuating the airfoil including a connection which yields substantially in proportion to the control force applied to the airfoil, and means operatively connected to said actuating means and to said adjusting means and responsive to such yielding of said connection to operate said adjusting means to increase the ratio of tab deflection to airfoil deection.

. HAROLD O. WENDT.

' REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS A Date Number Name 2,357,465 Focht Sept. 5, 1944, FOREIGN PATENTS Number Country Date 542,944 Great Britain Feb. 3, 1942 

